How can I access pre-existing UV maps in an fbx?
All existing UV maps can be accessed by simply selecting them in the Properties Editor, Data tab in the UV map list. They will be previewed in the viewport when selected, and you need to select the active one for rendering by clicking the camera icon:



When you need to use a few UV maps in a shader, you can specify the vector input for any texture to be the correct UV map with the node not so surprisingly called 'UV Map':